(1) Field of the Invention
The present invention relates to a turbocharger for an internal combustion engine, more particularly to a device for by-passing a part of the exhaust gas introduced into the turbocharger to limit the maximum intake air pressure.
(2) Description of the Prior Art
In a known turbocharger for a multi-cylinder internal combustion engine, the turbine housing is divided into a pair of juxtaposed scroll passageways. This type of turbocharger, called a "twin-entry type," enables effective use of the pressure wave generated in the flow of exhaust gas from the engine. The cylinders in this type of turbocharger are divided into first and second groups, so that the ignition strokes occur alternately between the two groups. These groups have respective exhaust manifolds connected to the respective scroll passageways in the turbine housing. As a result of this construction, exhaust ports of every two cylinders attaining two adjacent ignition strokes are separate from each other. Therefore, the pressure wave formed in each exhaust port is effectively maintained without weakening by the pressure in the exhaust port attaining the subsequent ignition stroke. The exhaust gases from the divided groups are independently introduced into the respective scroll passageways in the turbine housing.
All turbochargers, not only the twin-entry type, need a valve device (called a waste gate valve) for by-passing a part of the exhaust gas when the exhaust gas pressure becomes higher than a predetermined limit. In a twin-entry type turbocharger, this by-pass valve must be able to control the two divided passageways.
Japanese Unexamined Utility Model Publication (Kokai) No. 56-161138 discloses the provision of a pair of by-pass passageways opened to the scroll passageways. The by-pass passageways are selectively opened or closed by means of a common by-pass control valve arranged in the housing. In this prior art reference, however, since the by-pass passageways are opened directly to the scroll passageways, turbulence is generated in the flows of the exhaust gas in the scroll passageways. This decreases the efficiency of supercharging.
Another prior art reference, Japanese Unexamined Utility Model Publication (Kokai) No. 57-139635, discloses an exhaust by-pass system for a twin-entry type turbocharger wherein by-pass passageways are opened to the exhaust pipes at positions spaced from the scroll passageways. This prevents turbulence in the flows of the exhaust gas in the scroll passageways. In this utility model, however, the by-pass device is arranged outside of the turbine housing. This is disadvantageous in that it increases the dimensions of the device itself. Also, extra parts are required for the by-pass passageways located outside of the housing and the assembly of the device is complicated.